Liquid crystal displays (LCD's) are widely used as display devices in products such as word processors and personal computers, television sets, monitors, and mobile information terminals because they can be directly connected to an IC circuit, operated at low voltage and low power consumption, and formed into thin devices. The basic structure of the LCD is comprised, for example, of a polarizing plate provided on both sides of a liquid crystal cell.
Incidentally, the polarizing plate only allows light of a fixed direction in the polarizing plane to pass. The LCD thus has the important role of making visible the changes in the orientation of the liquid crystal caused by an electric field. In other words, the performance of the polarizing plate greatly affects the performance of the LCD.
The polarizer of the polarizing plate is one in which iodine and the like is adsorbed on a high molecular weight polymer film and is then extruded. That is to say, a solution, called H ink which includes a two-colored substance (iodine), is adsorbed by wet adsorption onto a polyvinyl alcohol film and then the two-colored substance is oriented in one direction due to uniaxial extrusion of the film. Cellulose resin, and cellulose triacetate in particular, may be used as the polarizing plate protective film.
Cellulose ester film is optically and physically effective as a polarizing plate protective film and is thus widely used. However, because the method for manufacturing the film is a casting method using a halogen-based solvent, the cost required to recover the solvent is an extremely large negative factor.
In recent years, as a manufacturing method of cellulose ester film for application to a polarizing plate protective film, a melt cast method has been carried out, for example in Patent Document 1. However, since cellulose ester is a polymer having a high glass transition temperature and a very high viscosity at molten state, a cellulose ester film, which is formed when cellulose ester is melted and extruded from dice to be cast on a cooling drum or on a cooling belt, is difficult to level, and solidifies in a short time after extruded. It has been found that there are problems in that such a cellulose ester film is poor in physical properties such as flatness, anti-curling and dimensional stability, and in uniformity of birefringence as an optical property, particularly in uniformity of birefringence in the transverse direction of the film, as compared with cellulose ester film obtained according to a solution cast method.
The melt cast method for manufacturing a cellulose ester film, comprising a heat application step (process) at a high temperature, has a significant problem that reduction of cellulose ester molecular weight due to thermal decomposition results in coloring of the film or in processing stability lowering of the film. When a cellulose ester film is manufactured according to a melt cast method, deterioration of cellulose ester proceeds due to mechanical stress applied while cellulose ester is heat melted at a high temperature and pelletized employing a uniaxial or biaxial extruder. Since the cellulose ester is further heated at a heat-extruding process until cellulose ester film is obtained, i.e., the cellulose ester is subjected to two time thermal history, deterioration of cellulose ester further proceeds. Although details are not clear, deterioration of cellulose ester results in coloring or lowering of processing stability of film probably because the molecular weight increases due to crosslinking or lowers due to depolymerization, resulting in a broad molecular weight distribution. This is a problem not only in product quality but also in reuse of cellulose ester film pieces occurring during manufacture for raw materials. Since cellulose ester is easily deteriorated by heat application, a melt cast method has been required in which cellulose ester does not deteriorate.
In order to solve such problems, there is proposed a method in which a hindered phenol compound, a hindered amine compound or an acid scavenger is added as a stabilizer in a specific amount to a cellulose ester film, thereby coloring or processing stability lowering of the film is minimized (see, for example, Patent Document 2.). Further, a technique is disclosed in Patent Document 3 in which a triazine compound or a benzoic acid phenyl ester compound is employed a plasticizer in order to prevent streak defects occurring after a melt cast film formation process taken for a long time. However, any known techniques as described above are still insufficient to overcome the above-described problems, particularly or processing stability lowering, coloring or lowering of uniformity of birefringence as an optical property.    Patent Document 1: Japanese Patent O.P.I. Publication No. 2000-352620    Patent Document 2: Japanese Patent O.P.I. Publication No. 2006-241428    Patent Document 3: Japanese Patent O.P.I. Publication No. 2006-176736